Understanding Plasticiser effects
Our observations and conclusions about plasticisers are based on field experience and from research into the chemistry of these products. We now never recommend plasticisers (or waterproofing and air entraining agents to be used above the damp course) as field tests indicate that even at 25% of recommended concentrations they still inhibit cleaning. The downside of plasticiser usage, for brick cleaners, is that even low amounts will cause problems as it waterproofs and hardens mortars excessively within a few days so acids cannot penetrate and react. Many Plasticisers also makes mortar stick much more strongly to the brickface. Unfortunately, much more acid and/or pressure is required to clean these plasticised mixes which leads to mortar bed and brick face damage. This problem is worsened tremendously if the bricks are not laid cleanly. The damage the combination of all these factors causes is evident in most of the mortar beds in most houses built in Victoria after pressure cleaning of bricks with acid began.
Plasticiser usage makes life easier for "Brickies" as it extends mortar board batch life in hot weather and makes it lighter, fluffy and more workable, but the downside is that it also waterproofs the mortar and makes it very hard, brittle and extremely sticky so when bricks are laid with plasticised mortars, they become very difficult to clean. Unfortunately, plasticiser usage has become common practice in Victoria and the local bricklayers try to blame the washed "sharp" Victorian sands for this trend.
Over time it appears that many Victorian bricklayers have become habitually reliant on plasticiser usage, regardless of whether not the sand was "sharp" or not and needed plasticiser to make mortar workable. Others may be using plasticiser to reduce the use of lime to increase their profit on jobs where they are responsible to provide lime. Also the practice of initially adding premixed water containing plasticiser to the mix (as recommended by plasticiser manufacturers) will cause a conflict with the properties of lime as the mortar will become too sticky in the mixer and make your mix unworkable if you try to adjust with lime. This may have contributed to the "addiction" to plasticisers some Brickies have. We work with many Bricklayers who never use plasticizer and see no need for it as they do not have this problem.
Why does this dichotomy of ideas exist?
Could it be the premixed plasticiser conflicting with lime!
We are doing field trials with one of Melbourne's volume builders to understand the consequences of modern mortar chemistry on cleaning. These results will be published once the experimentation is complete.
To avoid any problems it may be best to use plasticiser as a last resort and always test the cement mix to see if it doesn't harden excessively and can be cleaned. Using plasticisers could lead to hard and difficult to clean, low lime mixes that may not pass a mortar test as it could be deficient in calcium carbonate. It would also be excessively hard, sticky, brittle and have no elasticity.
Remember too that excessive amounts of plasticisers can also make mortar so hard that the bricks will crack, instead of the mortar if the slab moves during the curing process. Mortar made with lime was designed to be pliable and sacrificially crack (modulus of elasticity) instead of damaging the bricks. Mortar is meant to be self repairing (annealing) due to the lime content and in the worst case it is easier to fix by repointing cracked mortar beds rather than re-bricking a wall with cracked and damaged bricks. Recent research from the University of Dublin still concludes that Lime is the plasticiser of choice. To understand the problems better we have started to do the AS3700 scratch test on our jobs and are seeing the majority of mortars with hardnesses well in excess of a M4 Mix. The AS 3700 recommends M2 mortars and M3 in a harsher environment. M4 mixes are for severe marine environments or continuous chemical exposure. This hardness can only come from excessive cement or chemical additives. Field tests on mortar with most of the common additives eg lime replacements, plasticizers and waterproofing agents have also proved to us that these mortars become excessively hard and waterproofed within one or two days and then within 7 days we cannot clean them with B10K. Excessive pressure or large quantities of concentrated acid are required and this leads to the damage commonly seen in Melbourne. Based on the above field work and observations we never recommend lime replacements or plasticisers.
With the current trends for dark coloured face coated bricks laid with Brightonlite®cement, there is an increase in the number of brick faces and mortar beds destroyed by excessive applications of acid and the use of turbo's which are often needed to clean, dirty and poorly laid bricks with excessively hard mortar. If the brick layers had laid them cleanly using our 5 rules these problems would be hugely reduced. Builders too need to research thoroughly all the consequences of plasticiser use
Finally, it is interesting that the plasticiser manufacturers chemists that we spoke with were totally unaware of the problems caused by their products, but we also need to remember that the marketing departments may not allow the publishing or highlighting of any negative effects caused by their products.
For more see item number 5 onwards in presentation link below.